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IMO if you are dealing with red gills on the other fish, I would change the water more frequently (like twice a day - at least 50%) and leave out the meds - meds should really only be used for specific symptoms that point to a diagnosis.

If you notice the same issue with another fish, I would remove that fish to the QT and then ask for advice on what med to add.

IMO if you are dealing with red gills on the other fish, I would change the water more frequently (like twice a day - at least 50%) and leave out the meds - meds should really only be used for specific symptoms that point to a diagnosis.

If you notice the same issue with another fish, I would remove that fish to the QT and then ask for advice on what med to add.

Again, I am not trying to add meds for the red gills. I am trying to be ready TODAY if another of my loaches has symptoms similar to the first.

In other words, I AM trying to explain specific symptoms that point to a diagnosis so that I can understand why the one loach died yesterday. If it helps you to imagine that the loach is still alive and I am describing the symptoms in the present tense, then please do so.

I would like to be prepared if I have to deal with a recurrence of this problem today rather than have to spend time coming to a forum to try to coax an answer.

What I have learned so far is that Maracyn 2 is the best course if it is bacterial/fungal and that I should have PraziPro on hand as well. What I don't yet know is whether these meds should be reduced in dosage from the package instructions when administered to loaches.

Does anyone have direct experience of or knowledge about that to share?

The best preparation for issues like this is prevention. Prevention in your case is correctly stocking the tank and reducing the stress.

And that is a personal choice on your dosage amount.
some people hate meds and will try various natural methods, others swear by them and use the full dose.

what kevin said is perfect about the 2 meds that you have.

prazi pro is good for de-worming fish and recommended every time you qt new fish, and about once every 6-8 months in your main tank. it is a solid preventative med to add and doesn't have negative side effects.

I normally dose just under the normal amount because i'm not a big fan of meds.

again, to prevent this issue:
keep up with regular water changes to heal the gills of your fish.
fix the stocking to reduce the stress in the tank.

Thank you Kevin and Miz for the specifics about dosing. Hopefully, I won't have to administer anything, but I'm glad I have the info if I need to use it.

Everyone is swimming around peacefully after breakfast. I've seen only four loaches at one time, but I expect they'll all come out to play during the water change, as usual. (Water test was 0/0/5 again this morning, but I'll do a change anyway.)

IMO if you are dealing with red gills on the other fish, I would change the water more frequently (like twice a day - at least 50%) and leave out the meds - meds should really only be used for specific symptoms that point to a diagnosis.